The present invention relates to a semiconductor switch and, more particularly, to a semiconductor switch suitable to provide an overcurrent protection.
Electronic apparatuses need to be protected from an overcurrent. Two examples of the conventional protection circuits for the overcurrent have been disclosed in, "Semiconductor Handbook", published by Ohm Co., Ltd. 1977, pages 696 to 697. According to the current limiter type protection circuit, which is one of those protection circuits disclosed in this publication, in the series type DC regulating power supply using a transistor for control, a resistor to detect a load current is connected in series to a load. When a voltage across this resistor which is developed due to the load current exceeds a base-emitter voltage V.sub.BE of an overcurrent detecting transistor whose base and emitter are connected across this resistor, the overcurrent detecting transistor is made conductive and a base current of the controlling transistor is bypassed through a collector of the overcurrent detecting transistor, so that an increase in load current is prevented. However, according to such a current limiter type protection system, an overload current of a predetermined level continues to flow as long as the overcurrent lasts, resulting in an increase in heat generation of the controlling transistor. Therefore, it is necessary to set a heat capacity of the controlling transistor to a sufficiently high value. Consequently, in the case where such a current limiter type protection system is applied to the semiconductor switch, there is the problem that a chip area of the semiconductor switch is enlarged. On the other hand, according to the current shut-off type protection system, which is the other protection circuit described in the foregoing "Semiconductor Handbook", the overcurrent flows through the controlling transistor only during the period from the occurrence of the overcurrent condition till the complete shut-off by the controlling transistor, which period is small as compared with that of the current limiter type protection system, removing the heat related problem. Nevertheless, it is still necessary to considerably enlarge the chip area of the semiconductor switch in proportion to the magnitude of the overcurrent. Further, according to the current shut-off type protection system, after the current was shut off, the circuit must be reset by the external control circuit, so that there is also a problem that this results in an increase in number of parts and causes the cost to be raised.
A technology relating to the present application has been disclosed in Japanese Patent Unexamined Publication No. 14355/84 laid open on Jan. 25, 1984. This publication discloses turning off a gate turn-off thyristor (GTO) by short-circuiting the gate with the cathode of the gate turn-off thyristor by means of a transistor. However, nothing is disclosed with respect to the protection of the thyristor when the overcurrent is generated.
When a thyristor is to be used as a semiconductor switch, if the turn-off is performed at a value below a surge current withstanding capacity of the thyristor or below an allowable current square time product (I.sup.2 t), the thyristor will not be broken and the switch circuit and the system controlled by the switch circuit can be easily protected. However, there is a limitation on a turn-off current value and a particularly large thermal capacity or tolerance is needed to permit a large turn-off current. Therefore, it has been necessary to enlarge the chip sizes for the thyristor and short-circuiting transistor. In this connection, the turn-off current is defined as a current flowing through a thyristor at which current the thyristor can be safely turned off without being damaged.
In addition, an external circuit has been also necessary to detect the overcurrent and to supply a signal to turn off the thyristor.